LTSLEEP(9) BSD Kernel Developer's Manual LTSLEEP(9)NAME
ltsleep, tsleep, wakeup -- process context sleep and wakeup
SYNOPSIS
#include <sys/proc.h>
int
tsleep(wchan_t ident, pri_t priority, const char *wmesg, int timo);
void
wakeup(wchan_t ident);
DESCRIPTION
The interfaces described in this manual page are obsolete and will be removed from a future
version of the system.
The ltsleep() interface has been obsoleted and removed from the system.
Please see the condvar(9), mutex(9), and rwlock(9) manual pages for information on kernel
synchronisation primitives.
These functions implement voluntary context switching. tsleep() is used throughout the ker-
nel whenever processing in the current context can not continue for any of the following
reasons:
o The current process needs to await the results of a pending I/O operation.
o The current process needs resources (e.g., memory) which are temporarily unavail-
able.
o The current process wants access to data-structures which are locked by other pro-
cesses.
The function wakeup() is used to notify sleeping processes of possible changes to the condi-
tion that caused them to go to sleep. Typically, an awakened process will -- after it has
acquired a context again -- retry the action that blocked its operation to see if the
``blocking'' condition has cleared.
The tsleep() function takes the following arguments:
ident An identifier of the ``wait channel'' representing the resource for which the cur-
rent process needs to wait. This typically is the virtual address of some kernel
data-structure related to the resource for which the process is contending. The
same identifier must be used in a call to wakeup() to get the process going again.
ident should not be NULL.
priority The process priority to be used when the process is awakened and put on the queue
of runnable processes. This mechanism is used to optimize ``throughput'' of pro-
cesses executing in kernel mode. If the flag PCATCH is OR'ed into priority the
process checks for posted signals before and after sleeping.
wmesg A pointer to a character string indicating the reason a process is sleeping. The
kernel does not use the string, but makes it available (through the process
structure field p_wmesg) for user level utilities such as ps(1).
timo If non-zero, the process will sleep for at most timo/hz seconds. If this amount
of time elapses and no wakeup(ident) has occurred, and no signal (if PCATCH was
set) was posted, tsleep() will return EWOULDBLOCK.
The wakeup() function will mark all processes which are currently sleeping on the identifier
ident as runnable. Eventually, each of the processes will resume execution in the kernel
context, causing a return from tsleep(). Note that processes returning from sleep should
always re-evaluate the conditions that blocked them, since a call to wakeup() merely signals
a possible change to the blocking conditions. For example, when two or more processes are
waiting for an exclusive-access lock (see lock(9)), only one of them will succeed in acquir-
ing the lock when it is released. All others will have to go back to sleep and wait for the
next opportunity.
RETURN VALUES
tsleep() returns 0 if it returns as a result of a wakeup(). If a tsleep() returns as a
result of a signal, the return value is ERESTART if the signal has the SA_RESTART property
(see sigaction(2)), and EINTR otherwise. If tsleep() returns because of a timeout it
returns EWOULDBLOCK.
SEE ALSOsigaction(2), condvar(9), hz(9), lock(9), mutex(9), rwlock(9)HISTORY
The sleep/wakeup process synchronization mechanism is very old. It appeared in a very early
version of Unix. tsleep() appeared in 4.4BSD. ltsleep() appeared in NetBSD 1.5.
BSD January 28, 2012 BSD